1. Field of the Invention
The present invention relates to a method of manufacturing an optical semiconductor module for optical communication and in particular to a method of manufacturing a high-output optical semiconductor laser module for an optical fiber amplifier where high heat radiating ability is needed.
2. Description of the Related Art
In optical semiconductor devices, in particular in optical semiconductor modules such as optical semiconductor laser modules for optical fiber amplifiers, an optical semiconductor package is employed for hermetically accommodating an optical semiconductor, a driver IC, etc.
For example, as shown in FIG. 4, typically, in an optical semiconductor package, a frame 1 made of Fe/Ni/Co alloy (trade name KOVAR) is joined to a bottom plate 2 of, for example, Fe/Ni/Co alloy or Fe/Ni alloy (trade name 42 Alloy) or a composite metallic material, for example, CuW or CuMo. In particular, in a semiconductor package where power consumption is high, and high heat radiating capability is demanded, a CuW bottom plate is employed.
The frame 1 that constitutes the side walls of the semiconductor package is manufactured by machining or injection molding of KOVAR, and, in part thereof, is provided with ceramic terminals 3 which are constituted by a plurality of layers of ceramic sheet whose surface is metallized; a plurality of terminal leads 4 made of KOVAR are arranged projecting to the outside of the frame 1. Some optical semiconductor packages have a structure in which the side walls of the frame-are constituted of a ceramic insulator so as to be integrated with the ceramic of the terminals. Also, some optical semiconductor packages have a construction in which the terminal leads are glass-sealed into holes provided in the frame.
Also, a light transmitting window 5 for transmission of light between the inside and outside is formed in the frame 1 of the optical semiconductor package. The light transmitting window 5 consists of a pipe made of KOVAR, usually with a glass window member provided thereon to effect hermetic sealing. In some light transmitting windows 5 a glass window member is not employed but an optical fiber is passed therethrough and is hermetically sealed by soldering the optical fiber. In such cases, only the pipe of light transmitting window 5 is bonded to the frame 1 of the optical semiconductor package.
Assembly is effected by joining these components such as frame 1, bottom plate 2 and terminal leads 4, etc., by silver brazing, glass brazing, AuSn soldering or the like. The assembled optical semiconductor package as a whole is subjected to gold plating in order to perform final hermetic sealing with a lid, in order to prevent corrosion of the package, and in order to facilitate soldering in subsequent assembly step of the semiconductor module. A KOVAR ring is required for purposes of welding or soldering the lid on the upper surface of the frame 1 of the optical semiconductor package. Also, the melting point of the silver brazing alloy employed for assembly of the package is 600xc2x0 C. or more, the melting point of the glass brazing material is 500xc2x0 C. or more, and the solder for the window is AuSn, which has a melting-point of about 280xc2x0 C.
In order to package an optical semiconductor element in such an optical semiconductor package, an electronic cooling element is mounted on the bottom plate and then a circuit board on which an optical semiconductor element etc. have already been mounted are fixed thereon. In the electronic cooling element, N-type thermoelectric elements (for example BiTeSe) and P-type thermoelectric elements (for example BiTeSb) are arranged alternately in parallel, with the upper surfaces and lower surfaces of two respective adjacent thermoelectric elements being joined by a Pb60Sn40 solder having a melting point of 238xc2x0 C. to, the metal piece formed on the ceramic substrates. A Cu lead is connected to the outgoing electrode of the ceramic substrate, using a Pb90Sn10 solder having a melting point of 299xc2x0 C.
The electronic cooling element and the optical semiconductor package are then connected by means of a lead formed within the package, and the optical semiconductor element and the terminal leads of the optical semiconductor package are electrically connected by means of an Au wire. After sealing the cap onto the ring made of KOVAR, the optical semiconductor module is then manufactured by aligning the optical fiber in position on the light transmitting window of the semiconductor package and welding with for example a YAG laser.
In the electronic cooling element for an optical semiconductor module, metallizing is performed on the non-joined surfaces (the uppermost surface and lowermost surface of the electronic cooling element) of the ceramic substrate which is joined to the respective thermoelectric elements. For the joining between the lowermost surface of this electronic cooling element and the optical semiconductor package, a Pb37Sn63 solder having a melting point of 183xc2x0 C. is employed, and for the joining between the uppermost surface of the electronic cooling element and the optical semiconductor element, a low temperature solder having a melting point of 160xc2x0 C. or less, such as BiSn, is typically employed. These solder layers (which may also be referred to as xe2x80x9ca pre-solderxe2x80x9d) are laminated beforehand onto the electronic cooling element.
In an optical semiconductor module, in order to achieve optical coupling of the optical fiber and the optical semiconductor element, the optical semiconductor package and the electronic cooling element must be precisely positioned when soldering is effected. In order to achieve this, first of all the electronic cooling element is located in position by means of a die bonding tool, then heated to 183xc2x0 C. or more and joined to the bottom plate of the package using a Pb37Sn63 solder; after cooling, the die bonding tool is removed, and the optical semiconductor element is joined onto the top of the electronic cooling element using a low temperature solder such as BiSn.
However, there was the problem that when soldering the electronic cooling element onto the optical semiconductor package, the low temperature solder such as BiSn provided on the opposite surface (uppermost surface) of the electronic cooling element was melted, and oxidized. Consequently, slag was produced on the low-temperature solder such as BiSn, raising the thermal resistance of the solder joint when solder joining of the optical semiconductor element onto the uppermost surface of the electronic cooling element was effected. If the thermal resistance of this soldered joint is high, in the case of laser, the optical output of the optical semiconductor element such as an LD element is lowered, and, in addition, in the case of light-wave guide devices, the thermal uniformity becomes poor, widening the wavelength selectivity, in particular the half-value width, causing crosstalk.
In view of the above-mentioned prior art, an object of the present invention is to provide a method of manufacturing an optical semiconductor module wherein, when the electronic cooling element is joined to the optical semiconductor package, oxidation of the low temperature solder (pre-solder) provided on the uppermost surfaces of the electronic cooling element is prevented, and conduction of the heat at the soldered joint portion between the electronic cooling element and the optical semiconductor element is improved.
In order to achieve the above object, an object of the present invention is to provide a method of manufacturing an optical semiconductor module, comprising joining an electronic cooling element to a bottom plate of an optical semiconductor package and mounting an optical semiconductor element on the electronic cooling element, wherein the electronic cooling element is soldered to the bottom plate of the optical semiconductor package in a hydrogen atmosphere.
In the above-mentioned method of manufacturing an optical semiconductor module according to the present invention, the electronic cooling element may be provided with a PbSn solder on a ceramic substrate face on the side to be joined to the bottom plate of the optical semiconductor package, and a BiSn solder having a lower melting-point than that of the PbSn solder may be applied beforehand to the other ceramic substrate face on the opposite side where the optical semiconductor element is to be mounted.
With the method of the present invention, since, when the electronic cooling element is joined to the optical semiconductor package, solder joining is effected in a hydrogen atmosphere, even if the low temperature solder (pre-solder) for joining of the optical semiconductor element which is provided on the uppermost surface of the electronic cooling element melts, there is no possibility of its being oxidized. On the contrary, since oxide produced during bonding the low temperature solder to the electronic cooling element is removed, an excellent solder surface can be obtained on the low temperature solder on the uppermost surface of the electronic cooling element.
As a result, when the optical semiconductor element is soldered onto the uppermost surface of this electronic cooling element, no slag inclusion occurs, so the thermal resistance at the solder joint between the electronic cooling element and the optical semiconductor element is lowered, making it possible to improve heat conduction in this region. Consequently, the method of the present invention is suitable in particular for manufacturing a high-output optical semiconductor laser module for an optical fiber amplifier, in which high heat radiating ability is necessary.